Bigger engine on Mighty Mac

pawneepapa(6a KS)December 15, 2012

Given the high praise I have read on this forum for the Mighty Mac 12PT, I have been searching for one for the last few months. I just brought one home for a mere $100. It is in good shape, although it is missing the baffle door where the material exits the machine and the shield in the shredder hopper that keeps material from blowing back out. I ordered those parts from Mackissic and they emailed the manual (man they do customer service right).
The reason it went so cheap is the engine. The original engine died. It was replaced with a used 8hp Briggs from who knows what. It runs terrible because the float in the carb is broken (leaking gas and surging). Plus it has no throttle control. These are things I can fix but I would like to put a Honda engine on it eventually. So I have been looking for a replacement engine and found a used GX390 (13HP and 18.50 ft/lbs. of torque).

So here is my question. How much power can these machines handle. Mackissic now uses a Briggs with 11.5 ft/lbs. of torque on the residential version (and 13.5 on the commercial version). I talked to a tech at Mackissic and he suggested the GX240 (7HP, 11ft/lbs.) or GX270 (8hp, 13ft/lbs.), which seems conservative. But what do I know; mine is running so terrible I cannot tell if it is the leaves bogging it down or its just choking on gas.

Bottom line, what is the biggest engine someone has safely put on this machine?

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And the price to fix/replace the carburetor is how much?

The potential problem I see, is if you "overpower" it, instead of killing the engine if something "unchippable" goes in the hopper, you may simply break the machine beyond repair.

Sharp cutters are worth a couple horsepower vs dull ones.

    Bookmark   December 15, 2012 at 12:31PM
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My old MacKissic 12PT9 has a 9 hp Briggs and Stratton. The model before it was a 12PT8 with an 8 hp B&S and the model that came after mine was a 12PT10 with a 10 hp B&S. So MacKissic themselves are putting 10hp on the machine.

The engine drives a V-belt through a centrifugal clutch, which makes it easy to manually start the engine, because you are just turning the engine over and not the heavy flywheel/hammermill assembly. As the engine speed builds up, the centrifugal clutch starts to move the belt and turn the flywheel/hammermill assembly.

Because each of the 24 hammers is free to swing about its pivot, the hammermill is quite resistant to jamming, but if it should jam, the centrifugal clutch remains engaged because the engine RPM is still "up", so the V-belt starts to slip in the drive wheel, and friction will quickly heat the V-belt to the smoking point.

I have smoked my V-belt on many occasions, and I keep a new V-belt on hand as a spare. Smoking a V-belt doesn't necessarily ruin it. I have successfully used a V-belt after several smoking episodes. But when the belt stretches so much that you can no longer tension it by moving the engine forward in the mounting slots, you have no choice but to replace the V-belt. The bigger the engine, the more "smoke" you will get from a rotor jam.

The biggest risk of smoking a belt comes from stopping the machine with a full load of material in the hammermill chamber. That material will "set up" and the next time you start the machine, it will smoke the belt as soon as the engine RPM builds up enough to engage the centrifugal clutch. That takes just seconds.

I am happy with my 9hp B&S, because it is easy to start. However, my centrifugal clutch is now "on the fritz" and forcing me to pull both the engine and the rotor to get a start. Because of the ratio of the transmission between the engine and the rotor assembly, that isn't quite as hard as I thought it might be. But sometime when I have a lot of time on my hands (that may never happen) I will see if I can disassemble the centrifugal clutch and clean its guts. Or clean it guts without disassembling it. Or maybe just order a replacement part from MacKissic.


    Bookmark   December 17, 2012 at 10:23PM
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Harbor Freight engines are highly spoken of. They have an 11 and a 13 h.p. Appear to be clones of Honda, and much cheaper than Honda. With a cheaper engine, you might not feel so anxious about having something go wrong (as long as nobody gets hurt).

    Bookmark   December 17, 2012 at 11:05PM
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pawneepapa(6a KS)

I fixed the carb for $20 (new float and needle valve). It runs good now. But I discovered that the gas tank mounts are stripped and if you fill the tank more than 1/4 full it spits gas all over the engine, including the spark plug, not good. A new tank is $80, which seems silly when I only paid $100 for the whole machine. So I will keep looking for a used tank or a different engine, which ever comes along first.

Ideally, I would like to find a used Honda 11HP. 13hp engines seem much more common, I have already passed on two in the $200 to $250 range. Seems like the right decision after reading about Zenman's belt smoking on a 9hp. I'll bet 13hp would tear belts up quickly.

I have looked at the clone engines but the reviews on HF's website are mixed and at $300, I suspect that I can do better on a good used Honda.

I also noticed that a few of the hammers are slightly bent; look at the third hammer from the right in the picture. Do I need to pull it apart now to straighten them out, or can I wait until it is time to flip the hammers. The first corners are only slightly worn, so it may be a while.

    Bookmark   December 18, 2012 at 11:12PM
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Since you only paid $100 for a mighty pricey chipper, I'd say $80 for a good tank is small. As for the hammer, as long as it's functioning fine I wouldn't worry about 'til time to turn them.
You seem to be he!!bent on a bigger engine so you might as go ahead and get one at the risk of destroying a good chipper. Oh, and you might as well lay in a good supply of belts w/ the bigger engine.

    Bookmark   December 19, 2012 at 8:19AM
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Hi pawneepapa,

That bent hammer surprises me. My hammers are all straight, and they have been through a lot of use. Something strange must have gotten into the chamber to bend that hammer like that. My leading corners are rounded and I should rotate the hammers, but they are all straight.

You can avoid having a load of stuff set up and cause a belt smoke at the next startup by letting the chamber clear out pretty good each time before you shut the machine down.

I learned the hard way not to operate my machine at night. One evening I was trying to finish a pile of stuff I was shredding for a new a compost pile and dusk became dark and I could still see to feed my "Mighty Mac", so I continued chucking stuff into the feed hopper. Then, all of a sudden there was a loud noise, a terrible clatter, and fiery sparks and light coming out the holes in my screen and out the feed hopper as well.

I reflexively killed the engine and let it coast to stop. I made sure there were no fires in my product or the chamber, and waited until next morning to survey the damage and figure out what went wrong. I had to drop the screen to empty the chamber, and I found a great big rusty piece of scrap metal that had been marked up pretty good.

I was afraid that serious damage had been done, considering all the sparks and fire that had spewed from my machine, so I cleaned out the chamber and inspected the interior. There were some scratches and marks on the interior of the chamber, but I couldn't see any hammer damage or rotor damage, so I closed up the screen and did a short trial run and everything seemed fine.

I had been sweeping up old leaves, rotten twigs, and deadfall from the local forest floor and apparently that old piece of metal, possibly a bolt, was mixed in with that. On another twilight feeding occasion, a lemon-sized granite rock got in the chamber, causing a horrible noise and a quick shutdown. In a sense, that was worse than the bolt, because the swinging hammers did a pretty good job of breaking up the granite rock, and the fragments were almost as sharp as glass. I had to discard the product that was contaminated by the broken granite, because I didn't want that sharp stuff in my garden. Incidentally, breaking granite emits a little "fire", but nothing as brilliant as the steel bolt. That was a real fireworks show.

Hopefully I learned my lesson about not feeding my MacKissic in low light conditions. I still occasionally disintegrate a little rock while shredding, but limestone type rocks break up with no sharp edges or serious damage to the machine. I have heard that someone put oyster shells through a MacKissic hammermill to produce a product for his poultry flock, with the only ill effect being a deafening noise.

But I have to say, your bent hammer makes me wonder if someone didn't accidentally drop some kind of steel tool in your machine while it was running. Or start it up with a tool in the chamber. Anyhow, if it is working, don't fix it. You might straighten the hammer when you do flip the hammers at some later time. Did you get the manual for your machine? You are supposed to grease the main bearing fairly often -- every ten hours of use if I remember correctly. There should be a zerk that greases the main bearing.


    Bookmark   December 20, 2012 at 2:43AM
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pawneepapa(6a KS)

Here is an update on upgrading the engine on my Mighty Mac 12PT. I finally found a used 11hp Honda for sale but when I looked at it, I could tell right away that it wouldn't physically fit on the engine platform. The air cleaner sticks so far out to the left that even pushed up against the shredder box, two of the mounting holes would have hung over the edge of the platform.

So I gave up on the 11hp and went with a 9hp Robin Subaru Ex27. It just fit; I had to slightly move one of the angle brackets that goes from the shredder box to the platform to get an inch or so of room to tighten the belt. Of course the bolt pattern didn't match so I had to drill new holes.

But what a difference this engine makes. Before, 1.5" branches bogged down the engine. Now the machine can eat a 10' branch that is 2.5" on the thick end in less than a minute without significantly slowing down. I haven't touched the blade.

Same with leaves. Before a few handfuls of wet leaves bogged things down. Now it has enough power to keep spinning them until they squeeze through the screen. Without the screen, dry leaves just fly through without much shredding but wet matted leave go through quickly and shred up nicely.

I also got my clutch working properly by taking it apart and using emery cloth to remove the rust and scaling from inside the hub/pulley. With that fixed and the newer engine, its so much easier to start.

So all in all, it was definitely worth the effort. That machine starts and runs like a dream and it has extra power to get through the tough stuff. I have yet to smoke the belt.

If anyone is interested, I will take and post a picture.

    Bookmark   April 17, 2013 at 12:17AM
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Hi pawneepapa,

Congratulations on getting your 12PT bargain into excellent working order. Yes, we would like to see pictures. When I get some free time I will take my clutch apart and clean it.


    Bookmark   April 17, 2013 at 1:31AM
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pawneepapa(6a KS)

Sorry I am so slow posting a picture of the engine upgrade; I had surgery and my recovery has been slow.

Hopefully you can see in the picture how this 9hp engine completely fills the space from the grinding chamber to the edge of the engine platform. There is enough room but just barely. I did have to move the angle bracket; I left the extra hanging below the engine platform in case I ever want to move it back.

As mentioned in the posts above, this 9hp engine seriously out performs the 8hp briggs it replaced.

Zenman, thanks for your helpful responses. Hearing from someone with experience with this machine let me know that I could and should expect more out of it.

    Bookmark   May 14, 2013 at 11:16PM
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Hi pawneepapa,

You have showed a lot of practical ingenuity in completing the engine upgrade. It looks like a professional job. I notice that your older model came with some pretty small wheels.

I upgraded the wheels on mine to make it more stable and give a bit more room under the machine. The increased "footprint" makes it easier to pull it around.


    Bookmark   May 24, 2013 at 12:26AM
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